Rashi on TanakhPsalms 126

When the Lord returns the returnees to Zion from the Babylonian exile, we were like dreamers.

like rivulets in arid land Like rivulets in arid land, which moisten it, so shall we be moistened [freshened] when You return [us from] our captivity, for those who sow in an arid land, with tears, worrying that it will not grow, reap with song through the rivulets of water, when they are directed into it [that land].

He will go along weeping, etc. So do Israel in exile sow charity in tears, and they will reap it when You pay their reward in the future.